1. Field of the Invention
This invention relates to electrodeposited layers, and more particularly, to functional electrodeposited chromium layers having advantageous performance properties, and to a chromium plating bath and method for forming such useful chromium electrodeposits.
2. Description of the Prior Art
Hexavalent chromium plating baths are described in U.S. Pat. Nos. 2,750,337; 3,310,480; 3,311,548; 3,745,097; 3,654,101; 4,234,396; 4,406,756; 4,450,050; 4,472,249; and 4,588,481. These baths generally are intended for "decorative" chromium plating or for "functional" (hard) chromium electrodeposition. Decorative chromium plating baths are concerned with deposition over a wide plating range so that articles of irregular shape be completely covered. Functional chromium plating, on the other hand, is designed for regularly shaped articles, where plating at a higher current efficiency and at higher current densities is important.
Functional hexavalent chromium plating baths containing chromic acid and sulfate as a catalyst generally permit the deposition of chromium on a basis metal substrate at cathode efficiencies of about 12% to 16% at current densities of about 1 to 6 asi. Mixed catalyst chromic acid plating baths containing both sulfate and fluoride ions generally allow chromium plating at higher cathode efficiencies, e.g. of 22% to 26%, and at higher rates. However, the presence of fluoride ion in such baths causes etching of ferrous based metal substrates.
Other chromium plating baths which use iodide, bromide or chloride ions as additives can operate at even high current efficiencies, but such baths produce chromium deposits which do not adhere well to the substrate, and which are dull in appearance, or at best only semi-bright. For example, Chessin, in U.S. Pat. No. 4,472,249, describes a high energy efficient functional chromium electroplating bath which operates at very high current efficiencies, e.g. about 50%. These baths generally consist of chromic acid, sulfate, iodide, and a carboxylate, and baths are used at conventional current densities between about 1 to 6 asi. Unfortunately, this bath has adherence problems, poor low current density etching, and provides only a semi-bright deposit.
Chessin and Newby, in U.S. Pat. No. 4,588,481, describes a method for producing non-iridescent, adherent, bright chromium deposits at high efficiencies without low current density etching. This method involves plating at a temperature of 45.degree.-70.degree. C. from a functional chromium plating bath consisting essentially of chromic acid and sulfate, and a non-substituted alkyl sulfonic acid having a ratio of S/C of &gt;1/3, in the absence of a carboxylic or dicarboxylic acid.
Suzuki and Tsukakoshi, in U.S. Pat. Nos. 4,453,172 and 4,592,819, describe a very high speed plating apparatus for electroplating metals, e.g. chromium, within a very short time period. In this method, a flowing plating liquid is circulated at a high speed between a workpiece and anode in the plating chamber. The operating current densities permissible in such a system can range from 50-90 asi, which is an extraordinarily high current density, but which enables plating to occur very rapidly. In fact, the apparatus is referred to in the art as a "Rapid Plating System" (RPS). Unfortunately, the demands of this system necessitate a chromium plating bath which can operate under the extreme RPS conditions, and provide high performance chromium electrodeposits.
Accordingly, it is an object of the present invention to provide a high performance electrodeposited chromium layer, a chromium plating bath, and a method for forming such chromium electrodeposits, particularly under RPS conditions.
A specific object herein is to provide chromium electrodeposits which are adherent, bright, smooth, hard, wear resistant, exhibit a low coefficient of friction, and which can be formed at useful current densities, including both the very high operating densities of rapid plating systems, and the low current densities of conventional chromium plating.
These and other objects will be made apparent from the following more detailed description of the invention.